Because of the physical properties of silicon carbide, silicon carbide has been considered a likely semiconductor material for use in high temperature and high power applications. As a result, various semiconductor devices in silicon carbide have been developed in attempts to take advantage of the promising properties of silicon carbide. These devices include Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), Junction Field Effect Transistors (JFETs) and the ACCUFET.
In a power MOSFET, the gate electrode provides turn-on and turn-off control upon the application of an appropriate gate bias. For example, turn-on in an n-type enhancement MOSFET occurs when a conductive n-type inversion layer is formed in the p-type channel region in response to the application of a positive gate bias. The inversion layer electrically connects the n-type source and drain regions and allows for majority carrier conduction between source and drain.
The power MOSFET's gate electrode is separated from the conducting channel region by an intervening insulating layer, typically silicon dioxide. Because the gate is insulated from the channel region, little gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET from an on-state to an off-state or vice-versa. The gate current is small during switching because the gate electrode forms a capacitor with the MOSFET's channel region. Thus, only charging and discharging current ("displacement current") is required during switching. Because of the high input impedance associated with the insulated-gate electrode, minimal current demands are placed on the gate, and, therefore, the gate drive circuitry can be easily implemented.
Moreover, because current conduction in the MOSFET occurs through majority carrier transport only, the delay associated with the recombination of excess minority carriers is not present. Accordingly, the switching speed of power MOSFETs can be made orders of magnitude higher than that of bipolar transistors and thyristors. Unlike bipolar transistors and thyristors, power MOSFETs can be designed to simultaneously withstand high current densities and the application of high voltages for relatively long durations, without encountering the destructive failure mechanism known as "second breakdown" during switching transients. Power MOSFETs can also easily be paralleled, because the forward voltage drop of power MOSFETs increases with increasing temperature, thereby promoting an even current distribution in parallel connected devices. This is in contrast with devices relying on bipolar conduction such as the bipolar junction transistor or the thyristor where the on-state voltage drop is inversely proportional to the operating temperature.
The above-described beneficial characteristics of power MOSFETs are typically offset, however, by the relatively high on-resistance of the MOSFET's drift region for high voltage devices, which arises from the absence of minority carrier injection. As a result, a commercial silicon MOSFET's operating forward current density is typically limited to relatively low values, typically in the range of 40-50 A/cm.sup.2, for a 600 V device, as compared to 100-120 A/cm.sup.2 for the bipolar transistor for an identical on-state voltage drop.
A further limitation of MOSFETs in silicon carbide may also arise as a result of the MOSFET utilizing an inversion layer. As a result of the use of an inversion layer the low mobility of silicon carbide may result in high resistivities of the channel. Thus, the benefits of the advantageous properties of silicon carbide may be overshadowed by the limitations of the MOSFET device resulting from the utilization of an inversion layer and the low mobility of silicon carbide.
The ACCUFET was developed, at least in part, to overcome the limitations of the MOSFET. The ACCUFET uses separated buried base layers to protect the gate oxide from the base. The ACCUFET relies on an accumulation layer, rather than an inversion layer as in MOSFETs and, therefore, has a much higher channel mobility than a MOSFET in silicon carbide. The ACCUFET is further described in "The Planar 6H-SiC ACCUFET: A New High-Voltage Power MOSFET Structure" Shenoy et al., IEEE Electron Device Letters, Vol. 18, No. 12, December, 1997.
Furthermore, at high operating temperatures, the theoretical limits of a MOSFET or an ACCUFET may not be achievable because of gate oxide degradation by Fowler-Nordheim (F-N) current in the gate oxide. Fowler-Nordheim injection into the oxide may damage the oxide of the MOSFET and, ultimately, result in failure of the device by causing gate oxide breakdown. This breakdown may be further exacerbated by MOSFET structures which may have areas of field crowding at the oxide, for example at the corners of the oxide in the gate trench of a UMOSFET. See Agarwal, et al., Temperature Dependence of Fowler-Nordheim Current in 6H- and 4H-SiC MOS Capacitors, IEEE Electron Device Letters, Vol. 18, No. 12, December, 1997.
F-N injection or "hot electron" injection into the gate oxide may be even more problematic in semiconductor devices formed of silicon carbide because of the wide band gap of silicon carbide. This is because injection of inversion or accumulation layer carriers into the gate oxide is a function of the barrier height between the conduction band edge of silicon carbide and the conduction band edge of the gate oxide. Thus, it has been found that 4H-SiC with a band gap of 3.26 eV has a higher current density of F-N current than 6H-SiC with its bandgap of 2.85 eV. Agarwal, et al., Temperature Dependence of Fowler-Nordheim Current in 6H- and 4H-SiC MOS Capacitors, IEEE Electron Device Letters, Vol. 18, No. 12, December, 1997. This problem may be further exacerbated at high temperatures where the effective barrier height between the silicon carbide and gate insulator is reduced by statistical spreading in the carrier energy. Thus, devices which may appear extremely attractive in SiC may be limited by time-dependent dielectric breakdown as a result of F-N current in the gate oxide during both on-state and off-state operation of 4H-SiC MOS based devices such as the MOSFET and the ACCUFET.
An alternative to the MOSFET and the ACCUFET, the junction field effect transistor (JFET) may provide good gate control of current and voltage with a low on-state voltage drop. Furthermore, the JFET may be very reliable and provide good high temperature operation. Because the JFET does not have the semiconductor-oxide interface of the MOSFET and ACCUFET the breakdown of the oxide resulting from F-N currents may not present a problem. However, the JFET is a "normally on" device which may limit its applicability in many circuits. This is because the reliability of a power system may be compromised during gate drive failures. JFETs also suffer from relatively low voltage gains (ratio of drain voltage to gate voltage). Thus, a large gate bias may be required when the device is in the off state. The JFET's maximum breakdown voltage may also be limited by the gate-source breakdown voltage. Furthermore, the JFET may also have large leakage currents.
In light of the above discussion, there exists a need for improvements in high voltage power silicon carbide devices which offer a convenient gate control.